I had this same thing happen to me , I brought an engine with low miles, but it set for a few years. after it was in and running ,it to had a lot of blow by,you could put your thumb over the oil fill and it would build up pressure in the pan and blow out the oil as it ran down past the pan.It had good compression, one cyl was about 10-15 lbs lower then the rest.(135 145 PSI ) I put over 600 miles on it with verious quick fix oil additives and nothing seem to work. I have changed engines scence then and have not tore the old one apart yet but every one i know seem to think that it is stuck ringes. And i might add, it never smoked out the exhaust. so thats my two cents worth.